Hi everyone, I have a 97 SC400 with traction control, and I'm having a brake problem that I can't resolve. The brakes are soft, and travel about 2/3 of the way to the floor. Here is what I have tried:

I replaced the master cylinder, bled the brakes until all the old fluid was out and still had a soft pedal.

I replaced the front brake lines, and re-bled the system-no change.

I installed new pads, and re-bled the system-no change.

I disconnected the lines at the master cylinder and plugged them to see if the fluid was by-passing the piston cups inside the master cylinder- I couldn't get the pedal to budge ,so I guess that eliminates the master cylinder as the problem. There are NO LEAKS ANYWHERE!

I bought ANOTHER new master cylinder (Cardone brand again), took it to an established independent shop, they installed the new master cylinder, re-bled the system with a power bleeder- no change. The shop says they can't figure out what the problem is either.

I have worked on my own cars as long as I've been driving (~35yrs) and have done probably 60+ brake jobs including replacement of all components associated. I am stumped!

Does anyone have any suggestions? Is there something specific to this system that I am unaware of? Is the traction control system possibly causing this?
Also, does anyone have a source for traction control master cylinders? They are apparently almost impossible to find, and Lexus wants $900.00 for theirs

Any help is appreciated!

Regards,
M.R.

 

just a guess but maybe sticking caliper

or brake booster



or a bad vacuum line going to the brake booster

 

bryan767, thanks for the reply!
I don't think it's the booster or vac. to the booster. That would result in a HARD pedal, due to lack of assist. I have the opposite problem- so-o-o-o-ft pedal
The pistons are not sticking- I know because they retracted easily when I replaced the pads.

Thanks anyway-all suggestions appreciated!

Regards,
M.R.

 

Master cylinder need to be bench bled before installing into the vehicle. I know some shops won't even do that.

Other than that, chck to see if any of the calipers have visible leakage.
If not visible leak found at calipers, check brake booster leak.

Only time master cylinder needs replacement is when you detect a leak.
Look closely where brake pedal connects onto brake booster. If you see excessive amount of brake fluid on the rod that connects brake pedal, means booster is filled with Burke fluid n needs to be replaced. Due to brake fluid will eat through diaphragm in the booster.

Also need to check vacuum line. Sometimes, not sure if sc400 has it or not, there is check valve as well on the vacuum line that attaches to the booster.

 

jjpark712 thanks for the suggestions!
Again though, my problem is not one of not enough assist (as when the booster is damaged or is not getting vacuum), but the opposite- too soft a pedal, not too hard a pedal.

As for leaks- none. Not at the calipers, not at the booster to master cylinder junction (besides I replaced the original master cylinder with two replacements), not at the lines. There is never a detectable loss of fluid, nor puddles under the car.

This is driving me nuts! I'd really like to avoid taking it to Lexus and paying $2000 for a brake job on a $6000 car...

Keep 'em coming guys, I appreciate 'em all!

Regards,
M.R.

 

Any more ideas?

Anyone..?

Bueler...?

Anyone...?


Regards,
M.R.

 

Is this problem all of a sudden? Sc's don't have the best brakes and many of us do the ls400 big brake upgrade for the front. I got my reman'd caliper for 80$ a piece

 

you sure the calipers pistons are fine?

i recently had a buddy that had a stuck piston but it was compressed fine. after to rechecking it, he removed the caliper and had someone press on the brakes. they had to press on it 5-10 times before the piston moved.

BUT the problem he had cause his car to pull to one side whenever he braked.


soft usually mean the master cylinder but should have might at well changed the booster the second time you changed the MC.

 

When was the last time you've done brake fluid flush?? Need to flush out the system.
Some cars that I worked on has been neglected for several years that I tried to bleed brake system.
With pressure filling the system, when I crack open bleed screw, nothing comes out.
Even though it's sealed system, over time it will some how get contaminated and get dirty.

Also very important that when you are installing master cylinder, need to bleed it before installing on to the vehicle.

 

k7q- yeah the problem came on over a few days. The brakes were normal in feel and function, then over about a week they became softer and softer. I don't do much canyon-carving wit it so the normal brakes are adequate for my needs, if they are working properly.

jjpark712- I have put nearly 2 qt.s of fluid through the system, when replacing the master cylinders.

Both new master cylinders were bench-bled (one by me, one by the shop).

All four calipers have nice clear fluid, and the reservoir is clean because it is new.

All my pads (F&R) were low, but evenly worn with no "wedge" which would indicate a stuck piston.

All calipers were bone dry.


The shop said that the ABS unit is not a suspect for something like this, but I'm starting to wonder. Also,does anyone have a source for the hard-to-find traction control master cylinders?


Thanks for your replies,
M.R.

 

Possible pad knock-back? This can be caused by a worn out wheel bearing, basically making the rotor "wobble" pushing the pistons back in the caliper when moving, then when you go to push on the brake, the pistons/pads now have to travel further to make contact with the rotor face. Can you get the brakes to firm up by having the car running in neutral and pushing the brakes in and out without the car moving, or does it still feel soft?

Was the fluid ever low on the original master cylinder? I have heard that air can get stuck in the ABS module and can be a bit of a pain to get out. When you bled the brakes did you use the pedal to bleed it, or did you use an external pressure reservoir?

Good luck with the issue, I hate crap like this

 

scoobySC3- that's a good suggestion, and not something most people are aware of. I had that happen in an old Corvette I once had.
Unfortunately I get the same "sinking feeling" even when not moving. If I pump the brakes quickly 3-4 times it gets firmer briefly, but sustained pressure and they sink.

I'm leaning toward what you said about trapped air in the ABS unit. I used both the pedal method and a pressure bleeder, and the shop used a pressure bleeder to no avail.

Is there a technique specifically for bleeding the ABS unit?

I think I may just wuss-out and take it to the dealer, but it won't be till the end of the month. I'll let y'all know what the verdict is if I do.

Regards,
M.R.